1. Field of the Invention
This invention relates to an exposure system for photolithography in fabricating semiconductor devices such as LSIs.
2. Description of the Prior Art
During the fabrication of semiconductor devices such as LSIs, photolithography executes a process of transferring an image from a photographic mask to a resultant pattern on a wafer. Such photolithography is performed by an exposure system. Some exposure systems for photolithography are provided with a mask-to-wafer alignment apparatus.
Japanese published unexamined patent application 60-186845 discloses an exposure system provided with a mask-to-wafer alignment apparatus. In the exposure system of Japanese application 60-186845, a mask has alignment marks and a wafer has corresponding alignment marks. The alignment marks of the mask and the wafer are scanned by laser light, and the laser light scattered and reflected at the alignment marks represents the positional relation between the mask and the wafer. The laser light is analyzed to detect the error in the alignment between the mask and the wafer. The wafer is moved relative to the mask in response to the detected alignment error between the mask and the wafer to obtain a good alignment between the mask and the wafer.